1. Field of the Invention
The present invention relates to a method and an apparatus for measuring fluid flow using Doppler ultrasound techniques in general and in particular to a method and an apparatus for distinguishing ultrasound signals returned from moving gas bubbles or solid particles in a fluid from signals due to ultrasound transducer motion.
2. Description of the Prior Art
Doppler ultrasound has been used for many years to measure the flow rate of fluids that contain particles which reflect sound energy and shift the frequency of this sound in a direction and by an amount proportional to the direction and velocity of movement of the particles relative to an ultrasound transducer. See, for example, P. T. Wells and R. Skidmore, "Doppler Developments in the Last Quinquenium", Ultrasound in Medicine and Biology, 11:p. 613-623, 1985.
When a reflected frequency modulated ultrasound signal is received, it is typically converted to a lower frequency, e.g. d.c., to remove the carrier and provide an audio baseband signal. The baseband signal is then digitized. A Fourier transform of the digitized baseband signal is then used to produce a spectrum of the power at each frequency therein. The spectrum, which contains both positive and negative frequency components corresponding to changes in the velocity of the fluid flow in a forward and a reverse direction, is then used to distinguish flow towards the transducer from flow away from the transducer.
Referring to FIGS. 1 and 2, there is shown, respectively, a typical power spectrum output from a spectrum analyzer for cases of forward fluid flow (flow toward the transducer) and reverse fluid flow (flow away from the transducer). Forward flow corresponds to signal energy in the positive frequency portion of the power spectrum; i.e., the signal to the right of the vertical center line in FIGS. 1 and 2. Similarly, reverse flow is represented by the negative frequency portion of the power spectrum--the signal to the left of the center line in FIGS. 1 and 2. The amplitude of the power spectrum at each frequency corresponds to the percentage of the total volume of fluid flowing at a particular velocity. For example, as shown in FIG. 1, the highest percentage of the total volume of fluid is flowing toward the transducer at a velocity corresponding to frequency f.sub.1 and a substantially equivalent percentage of the total volume of fluid is flowing toward the transducer at a velocity corresponding to the frequency f.sub.2. In FIG. 2, the highest percentage of the total volume of fluid is flowing away from the transducer at a velocity corresponding to the frequency f.sub.3. The spectrum to the left of the centerline in FIG. 1 and to the right of the centerline in FIG. 2 is mainly noise.
When a gas bubble (or solid particle) passes in front of an ultrasound beam, the discontinuity in acoustic impedance at the gas-fluid interface (or particle-fluid interface) causes a strong reflection which can be detected by the ultrasound receiver.
Referring to FIG. 3, there is shown a typical power spectrum of a Doppler signal from a gas bubble. To the right of the centerline there is a large amplitude signal showing a flow of the bubble in a forward direction. The existence of such strong reflections due to gas bubbles or particles has been noted by Spencer et al in an article entitled, "Detection of Middle Cerebral Artery Emboli During Carotid Endarterectomy Using Transcranial Doppler Ultrasonography", Stroke, Vol. 21, No. 3, March 1990, p. 415-423, where it was reported that such emboli can be recognized by the strong signal returns they generate, and by a characteristic chirping character of the received signal. The chirping described implies that the returned signal shows a continuous change in velocity of the strong reflection from the bubble.
Previously, Padayachee et al, in an article entitled, "Computerised Techniques for Detecting Gaseous Microemboli in Blood Using Pulsed Doppler Ultrasound", Perfusion, 2:213-218, 1987, described two methods of bubble detection: one which employed the high amplitude of the return from a bubble, and one which used the broad spectral shape of the return which is due to the overload of the Doppler electronics from the strong response.
When a medical Doppler transducer is moved on the skin, large return signals, similar to those obtained from emboli, are generated.
While Padayachee et al, supra, recognize the high amplitude returns generated from embolic events, no mention is made of any apparatus or method for distinguishing these returns from returns due to probe motion. On the other hand, Spencer et al, supra, describe an ability to audibly distinguish between emboli (bubbles) and probe motion artifacts. However, there does not appear to be any clear quantitative criteria given by which such distinctions can be made mechanically and automatically. For example, as published by Spencer et al in their article, signals in spectrograms indicative of probe motion are not readily distinguishable from signals seen in the spectrograms of returns due to emboli. Both appear to exhibit high amplitude, broad spectrum signals with concentrations of energies in the low frequency ranges toward zero. Thus, if a bubble detection apparatus is limited to simply monitoring the returned Doppler signal for large amplitude signals, the apparatus would falsely report all probe motion artifacts as bubbles or particles. These false alarms would seriously impair the use of the apparatus for monitoring applications, since the users would lose confidence in the accuracy of the apparatus.